1. Field of Invention
This invention relates to a graphic display system which can be effectively used to provide a man-machine interface for process control; and more particularly, to such a system comprising a plurality of graphic display units which are connected to a host processor through a common bus, wherein each graphic display unit receives drawing commands and display commands sent from the host processor and displays in parallel graphics and patterns in compliance with the characteristics of information on a display unit, such as a CRT.
2. Description of the Prior Art
A graphic display unit , which is used to provide a man-machine interface for process control, is required to effectively display a variety of information at high speed on a display means for managing and controlling the plant as a whole.
FIG. 1 depicts a prior art graphic display unit wherein a host processor 1 is connected to a common bus BS; graphic display/keyboard controller 2.sub.l . . . 2.sub.n, enables the function of a man-machine interface to be connected to common bus BS and includes therein an extensive subprocessor 3. Common bus BS is also connected to an interface of a disk unit and a communication interface 6.
When host processor 1 controls controllers 2.sub.l . . . 2.sub.n, (1) the controller reads, as the bus master, commands written in the command block of main memory 11 of host processor 1, in accordance with instructions from host processor 1, or (2) host processor 1 tries to obtain a bus right within the controller and transfers a command train by directly writing the commands in internal memory 20.
However, in the first (1) case, the controller has to become a bus master. Thus, the required configuration is complicated and overhead by transfer of bus right increases. This results in decreased performance drop which is no longer negligible.
In addition, the second (2) case lowers remarkably the processing speed of the subprocessor located within the controller because the bus right is lost.